ABSTRACT While rapid identification of pathogens, novel therapeutic interventions, and passive immunization have critical roles in disease control, none can substitute for pre-existing protective immunity. Mucosally delivered bacterial live vector vaccines represent a practical and effective strategy for immunization. In live vector vaccines, genes that encode protective antigens of unrelated pathogens are expressed in an attenuated vaccine strain and delivered mucosally to generate relevant local and systemic immune responses. The broad hypothesis of this research plan is that by appropriate manipulation of novel Salmonella enterica serovar Typhi live vector platform technologies, we can construct a mucosally administered bivalent vaccine against potentially lethal infections caused by Acinetobacter baumannii. This unique research plan is designed to remodel the outer membrane of an attenuated S. Typhi-based live vector vaccine into an antigen presentation platform in which protective outer membrane antigens from A. baumannii are mucosally delivered to immune inductive sites via a novel inducible outer membrane vesicle delivery system. Mucosal delivery of recombinant outer membrane vesicles (rOMVs) via live vector vaccines offers significant advantages over conventional acellular OMV-based vaccination strategies including: 1] sustained in vivo delivery to mucosal inductive sites, and 2] delivery of rOMVs enriched in properly folded protective antigens. To implement this unique vaccination strategy, we will engineer synthetic genes encoding two protective outer membrane proteins, AbOmpA and AbOmpW, expressed from cassettes stably integrated into the chromosome of our live vector vaccine. To enhance delivery of these protective antigens to immune effector cells, thereby improving the protective efficacy of this mucosal vaccine, we will enhance delivery of rOMVs carrying AbOmpA and AbOmpW through inducible over- expression of the hypervesiculating proteins ClyA or PagL. The immunogenicity of these optimized live vector strains will be determined using a murine intranasal model of immunogenicity; efficacy will be evaluated after challenging immunized mice with a hypervirulent A. baumannii clinical isolate, proven to be lethal to nave mice within 48 hours post-challenge by either the systemic or intranasal routes of challenge. Success with this proposal will produce a bivalent mucosal live vector vaccine, effective against potentially lethal systemic and pulmonary infections with A. baumannii, which could prove highly valuable in both civilian and military settings.